The present invention relates to transmission diversity techniques used in the field of spread spectrum radio communications. It applies in particular to radio communications with code division multiple access (CDMA).
A transmission channel between a transmitter provided with n transmission antennas and a receiver provided with m reception antennas is considered. A spreading code c(t), consisting of a periodic sequence of complex samples called “chips” of rate fc, is allocated to this channel. It serves to modulate n sequences of complex symbols si(t) (1≦i≦n) having a symbol rate fs that is smaller than fc. The ratio SF=fc/fs is the channel spreading factor. The signal yj(t) picked up by the j-th reception antenna (1≦j≦m) may be written:
                                          y            j                    ⁡                      (            t            )                          =                                            ∑                              i                =                1                            n                        ⁢                                                            s                  i                                ⁡                                  (                  t                  )                                            ⁡                              [                                                      c                    ⁡                                          (                      t                      )                                                        ⊗                                                            h                      ij                                        ⁡                                          (                      t                      )                                                                      ]                                              +                      w            ⁡                          (              t              )                                                          (        1        )            where x designates the convolution operation, and w(t) designates the white noise and Gaussian noise. For one and the same user, the same spreading code is used on the various transmission antennas. In a CDMA system, the noise w(t) contains contributions pertaining to other users of the system.
The impulse response hij(t) of the propagation channel between the i-th transmission antenna and the j-th reception antenna is conventionally estimated by the receiver by virtue of known pilot sequences transmitted respectively by the n transmission antennas. It is generally modeled as a set of p paths taking into account per pair of antennas (p≧1), the k-th path (1≦k≦p) corresponding to a reception delay τk and to a complex reception amplitude aijk. Each propagation channel (i-th transmission antenna to j-th reception antenna) is thus associated by the receiver with a vector of p amplitudes: Aij=[aij1, aij2 . . . aijp]T, (the notation [.]T designates transposition).
The demodulation in a spread spectrum system consists in despreading the signal received at the level of each echo, by correlating the signal received with the spreading code. The receiver most commonly used is the “rake” receiver, in which the signal emanating from each antenna j is subjected to a filter matched to the spreading code whose output is sampled at the instants corresponding to the p paths identified. This provides a vector Z=[z11 . . . z1p . . . zm1 . . . zmp]T, where zjk designates the output of the matched filter relating to antenna j, sampled with the delay τk. Thus, at a given symbol time, the following system of equations is obtained:Z=HS+N  (2)where
  H  =      [                                        A            11                                    ⋯                                      A                          n              ⁢                                                          ⁢              1                                                            ⋮                          ⋰                          ⋮                                                  A                          1              ⁢                                                          ⁢              m                                                ⋯                                      A            nm                                ]  is a matrix representative of the overall channel, of mp rows and n columns;                S=[s1 . . . sn]T is a vector containing the n symbols transmitted at the time considered from the n transmission antennas; and        N is a noise vector of size mp.        
The system (2) is of a form very commonly encountered in signal processing. It is easily solved by a conventional least squares estimation procedure (MMSE, “minimum mean squared error”) on condition that the rank of the matrix H is at least equal to n. The MMSE solution may be written:Ŝ=(H*H)−1H*Z  (3)
Assuming that the antennas are not perfectly correlated, the rank of the matrix H is generally equal to the minimum of the integers n and mp. The necessary and sufficient condition to be able to solve the system (2) by the MMSE procedure is then mp≧n. Once this condition is satisfied, it is possible to solve the system according to the desired technique, by the MMSE procedure or by another procedure such as for example maximum likelihood sequence estimation (MLSE, this MLSE procedure may also be applied when mp<n, but it is then very unstable and sensitive to noise).
The performance of the receiver depends on the conditioning of the matrix of the channel H, which depends on the number m of reception antennas, the number p of paths and the decorrelation properties of the antennas. Correlated antennas cause poor conditioning due to the fact that the matrix H*H then has eigenvalues close to zero which disturb its inversion in the solution according to (3). In general, the designer of a radio station with multiple antennas contrives matters such that they are decorrelated, by spacing them sufficiently far apart and/or by making them radiate according to different polarities.
In the known systems with multiple inputs and multiple outputs (MIMO), i.e. with n≧2 and m≧2, one seeks to increase the accessible communication throughput for a given transmitted power, by transmitting different symbols s1, . . . , sn through the n transmission antennas. These symbols may be mutually correlated, if they emanate from a space-time coding, or independent. To definitely comply with the condition on the rank of the matrix H, the receiver should be equipped with at least n reception antennas. Otherwise the system (2) would be insoluble in the presence of a single path.
Examples of such MIMO systems are described in EP 0 817 401, EP 0 951 091, EP 1 117 197, WO 99/14871 and WO 99/45657.